1. Field of the Invention
The present invention relates to a gasoline engine. More particularly, the present invention relates to a compression ignition gasoline engine using low-cetane number fuel, such as gasoline.
2. Description of the Related Art
In general, a gasoline direct injection engine equipped with a solenoid-typed in-ward injector and two-stage variable valve lift for maximizing the internal ERG is applied to gasoline Controlled Auto Ignition (CAI) engines in order to generate self-ignition, and the fuel is injected one time in a recompression process or two times in the recompression process and an intake process.
In the two-time injection, as compared with the single injection, the fuel efficiency is considerably improved and the combustion pressure ascension rate (dp/dθ) is low in accordance with load conditions; however, the combustion pressure ascension rate (dp/dθ) is relatively large, at 4 bar/deg or more, without a difference above predetermined load.
The combustion pressure ascension rate (dp/dθ) represents a rapid raise of combustion pressure and is used as an index indicating representation of combustion noise.
The increase of the rate significantly increases the noise level that a driver feels and causes a problem in engine durability.
For reference, comparing with the compression ignition diesel engine, in terms of combustion noise level, the combustion pressure ascension rate (dp/dθ) of common diesel engines is about 2-3 bar/deg, and about 4-5 bar/deg when the maximum output is required. Accordingly, it can be seen that the level of combustion pressure ascension rate (dp/dθ) of current gasoline CAI engines is very high.
The fuel injection method for implementing a gasoline CAI is divided into an MPI type that injects fuel to the intake port and a direct injection type that directly injects fuel into the cylinder.
The MPI type was applied in the earlier state of development, but recently, the direct injection type that makes it possible to inject fuel in various ways is developed.
The injector used for the direction injection type is a solenoid-typed in-ward injector, which has a structure in which an injector pintle moves up in the injector in fuel injection and the fuel is injected through a hole formed through the injector.
The injector is disposed at the center of the combustion chamber under the intake port, which is a center injection type.
The center injection type has the advantage of minimizing that the fuel injected in the recompression process hits against the upper portion of the piston.
However, since the solenoid-type injector is small in injection rate, 14-15 g/sec, the number of multiple injection is limited when it is required to inject the fuel several times for a short time, and particularly, it may cause a problem, such as coaking.
Since the gasoline CAI engine basically uses self-ignition for combustion, combustion is generated simultaneously at several positions in the combustion chamber.
In this case, unburned fuel generates soot and the injection hole of the injector is clogged with the soot. This phenomenon is referred to as coking.
The coking makes wrong changes in the injection amount and shape of the injector, and as a result, the combustion becomes unstable and the engine fails.
Therefore, it is impossible to perform compression process injection that generates combustion by a limit in selection of injection time due to the problem and it is also difficult to perform multiple injection above two times.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.